The clinical usefulness of the preoperative platelet count and serum CA 125 level for predicting malignancy in patients presenting with a pelvic mass was investigated. A retrospective review of medical charts of 300 patients presenting with a pelvic mass between January 1995 and December 1997 was performed. Patients were divided into groups by pathologic diagnosis including ovarian cancer (n=20), benign ovarian tumor (n=60), endometrioma (n=20), uterine myoma (n=150), and adenomyosis (n=50). Thrombocytosis and elevated values of serum CA 125 level were defined as platelet count > 400,000/ul and 35U/ml, retrospectively. The x2 test was used to analyze the data. The total number of case of ovarian carcinoma, benign ovarian tumor, and endometrioma were significantly correlated with thrombocytosis(P=0.001). The tumor type and the serum CA 125 level of benign ovarian tumors were also significantly correlated with thrombocytosis(P=0.032, P=0.009). Thrombocytosis was found in 25% of serous cystadenoma cases and in 30% of dermoid cyst cases but was not present in any case of mucinous cystadenoma. There was no significant correlation between thrombocytosis and clinicopathological factors, such as type, stage, and serum CA 125 level of ovarian carcinoma(P>0.05). Also, myoma, adenomyosis and endometrioma were not significantly correlated with thrombocytosis. However, in 19.8% of intramural myoma cases, 46.1% of submucosal myoma cases, and 36.3% of subserosal myoma cases, thrombocytosis was present(P>0.05). The platelet count is a test that is rapidly available and easily obtained. Thrombocytosis is apparantly a marker of tumor burden, but is a little value in planning the original management of women presenting with a pelvic mass.